1. Field of the Invention
The present invention relates, in general, to a method for coating a photoresist film useful in the fabrication of a semiconductor device and, more particularly, to a photoresist film which has a uniform thickness, thereby improving the production yield and reliability of a semiconductor device.
2. Description of the Prior Art
High integration of a semiconductor device is greatly indebted to the development of processes for forming fine patterns. In particular, photoresist film patterns formed by lithography processes are extensively used as masks for etch or ion-implantation processes in the fabrication processes of semiconductor devices. Accordingly, for better photolithography, fine patterning of photoresist film, security of processing and clear removal of the photoresist film pattern are necessary. In addition, when a photoresist film pattern is incorrectly formed, re-establishment of the photoresist film pattern should be accomplished with ease.
A typical photoresist film pattern is formed by a method consisting broadly of: mounting a wafer on a spinner; feeding a photoresist solution comprising photoresist agent and resin onto the wafer; rotating the spinner in high speeds, to form a uniform coating of the photoresist solution on the wafer; illuminating a beam of light on the photoresist coating by use of an exposure mask having a light screen pattern, to polymerize the photoresist coating; and developing the illuminated photoresist coating, to remove the unilluminated areas of the coating.
In order to better understand the background of the present invention, conventional techniques for forming photoresist film will be described with reference to FIG. 1.
Referring to FIG. 1, there is illustrated a prior technique for forming a photoresist film on a very rugged surface of a wafer. On a spinner 10 is mounted a wafer 11 on which insulation film patterns 13 capped with conductive wiring 15 are covered with a blanket interlayer insulation film 17 of oxide or nitride, as shown in FIG. 1. As a result, there are many prominence and depression parts on the wafer 11. While the spinner 10 is rotated at a high speed, a negative or positive photoresist solution is fed onto the wafer 11, to form a photoresist film 19 of smooth surface. Accordingly, the photoresist film 19 is relatively thick (with a thickness of t.sub.1) at the prominence parts attributable to the conductive wirings 15. On the other hand, the photoresist film 19 is relatively thin (with a thickness of t.sub.2) at the depression parts, as shown in FIG. 1. By reference, the higher is the degree of integration of a semiconductor device, the more rugged the surface of the wafer 11 is by such patterns as the conductive wirings 15.
When the photoresist film formed by the prior technique, of which the thickness is different from one portion to another on a wafer, is illuminated with a beam of light, the energy absorbed by the photoresist film is not uniform by its portions because of its different thickness. Therefore, the finished photoresist film patterns are different in size from one another, leading to a decrease in the uniformity of critical dimension.
As mentioned previously, the photoresist film of a semiconductor device according to the prior technique has different thicknesses due to intense ruggedness or topology change of a substructure, resulting in a difference in absorption energy of light. Accordingly, since the final patterns are not similar to one another, the production yield and the reliability of a semiconductor device are lowered.